The purpose of this research is to quantitatively define the role of physical and mechanical parameters in the microcirculation that influence and determine whole-organ regulatory and exchange functions. Three major experiments are planned. The first experimental goal will be to record, in vitro, the length-tension characteristics of single 1st, 2nd, 3rd and 4th-order arterioles isolated from hamster cheek pouch. The modified Burg technique developed by Duling et al will be used. The specific question that will be asked is whether smooth muscle in different segments of a given microvascular bed has quantitatively the same length-tension characteristics, in vitro. The answer to the question will provide an essential test of the validity of an hypothesis proposed previously by the principal investigator that explains in simple mechanical terms why there are regional differences in the response of microvessels to vasoactive agents, in vivo. The key assumption in the original hypothesis was that smooth muscle cells in different regions of the vasculature operate on the same length-tension curve. The second goal will be to characterize the veno-arteriolar reflex response in the microcirculation of rat intestinal muscle, submucosa and mucosal villi. The modified Gore-Bohlen preparation will be used. The purpose of the experiments is to discern the mechanisms of the arteriolar reflex response to venous pressure elevation. Specific questions that will be asked are: Is the response neurogenic? Is it myogenic? Can the reversed gradient in the veno-arteriolar response be explained in terms of a reversal in the normal stress gradient? The third experimental goal will be to do a comparative measurement of single capillary hydraulic conductivities (Lp's) in avian, mammalian and amphibian skeletal muscle to answer the question why Lp's in avian skeletal muscle capillaries are so large. Four questions will be explored. a.) Is the large Lp due to a systematic error in the methods used (Lee versus Michel)? b.) Is it due to capillary compliance? c.) Is it due to the high temperature at which birds normally operate? d.) Is it because of known differences in avian plasma protein composition that may determine the nature of the exchange barrier? These experiments will settle several questions about methods, compliance, temperature and the effect of protein on Lp.